Such a system comprises sources which produce various bit streams which it is to protect differently against the errors due to imperfections of the transmission channel. The tolerated error rate depends on each bit stream. For example, a source may produce three bit streams which it is to transmit with the respective maximum error rates of 10.sup.-4, 10.sup.-7 and 10.sup.-11.
A customary method for having a selective protection comprises utilizing different codes for each bit stream. The transmitted signal is then obtained by a time-division multiplexing of the various coded signals coming from various bit streams. For example, error correction codes may be used whose correction capacity is adjusted as a function of the required error rate. This approach has two drawbacks:
the decoding requires a different decoder for each bit stream, leading to much hardware complexity; PA0 the coding does not depend on the modulation used, which does not provide optimum performance in terms of error rate as a function of the signal-to-noise ratio. For an error rate and a fixed rate, the signal-to-noise ratio is not minimized. PA0 compared with a system using the binary punctured codes with a PSK4 phase modulation, a twice higher transmission capacity is provided (spectral efficiency between two and four bits/s/Hz); PA0 compared with a system using the conventional punctured codes associated to a QAM modulation, there is a more optimized performance in terms of error rates; PA0 the system does not have much hardware complexity because a single Viterbi decoder is necessary to decode the various data streams which correspond to the various protection levels; PA0 the codes used are preferably 2/4 convolutional codes with two input bits and four output bits. The function generating the four output bits in response to the input bits and state (memory) of the coder has been optimized in view of the QAM16 modulation; PA0 the puncturing is applied to 4-AM symbols, that is to say, real symbols assuming four possible values (+1, -1, +3, -3) and not to bits as is the case with the prior-art puncturing; PA0 the convolutional codes having a 2/4 rate are chosen in such a way that the minimum euclidian distance of the code 2/4 combined with the QAM16 modulation is maximized. The same holds for the minimum euclidian distance of the punctured code 2/4 combined with the QAM16 modulation. This makes the combination of the codings come under the heading of the modulation.
The first drawback has been suppressed in the systems utilizing the punctured codes described in the article: "Rate-compatible punctured convolutional codes (RCPC codes) and their applications", IEEE Transactions on Communications, J. Hagenauer, vol. 36, No. 4, April 1988, pp. 389-399. The principle consists of utilizing a single convolutional code (called mother code) having a 1/2 rate or 1/3 rate, the same for all the bit streams, and of applying a different punctured coding for each stream. The punctured coding consists in this article of periodically prohibiting the transmission of certain bits as a function of a matrix, called puncturing matrix. Thus, the rate of the punctured coding becomes higher than that of the non-punctured coding. This document relates to PSK2 or PSK4 phase modulations.
At the receiving end, the decoding is performed by computing metrics of the symbols received in the baseband coming from each bit stream. The computation of the metrics depends on the puncturing matrix, thus on the bit stream. The transmitted bits are estimated by a Viterbi decoder shared for the processing of all the signals that correspond to the various bit streams.
The drawback of the codes having a 1/2 rate is that they are only adapted to low-efficiency spectral modulations (less than or equal to two bits/s/Hz) as PSK2 and PSK4 phase modulations. Corresponding to these modulations is a bit rate equal to the maximum of twice the band used. For increasing the rate for a fixed seized band, one could seek to utilize high-efficiency spectral modulations of the QAM type (Quadrature Amplitude Modulation). But the use of the punctured convolutional codes having a 1/2 rate described by J. Hagenauer placed side by side with QAM modulations does not yield optimum performance, because these codes have been devised independently of the modulation.